Rhodo.JPG (23040 bytes)Rhododendron ponticum: Australia has become home to many plants from overseas. Many of these exotics have become noxious and obnoxious weeds. Blackberry and Bitou Bush are two introductions that are having great economic and environmental impact in Australia. There is a two-way traffic in weeds, with Australia exporting plants that are not a problem here but have caused immense problems in their adopted homes. Casuarinas and Melaleucas in Florida and Acacias and Hakeas in South Africa are examples.
The United Kingdom is not usually thought of as having noxious weed problems but there is one plant that is causing great environmental problems.
Rhododendrons are cultivated in many areas of Australia and as far as we know there are no weed problems with the genus. In the United Kingdom, with one species, this is a different kettle of fish. Rhododendron ponticum was introduced into Britain in the Victorian era. During this period plant collectors were scouring the world for unusual and colourful plants to introduce into British gardens. Rhododendron ponticum was one of these plants and occurs naturally in Spain, Portugal and extends into China. It is a small tree or tall shrub with attractive deep lilac flowers was cultivated on country estates for its ornamental value and as a cover for game birds.
The 20th century saw the demise of many of these country estates and this has led to a lack of control of Rhododendron plantations. This has allowed the species to invade large areas of British countryside. Given the right conditions, Rhododendron ponticum has many of the characteristics that allow it to become an obnoxiously noxious weed.
Rhododendron ponticum has a dense leaf canopy that shades the ground and inhibits the establishment of competing native plants. This leads to a loss of native animal and bird habitat.
Rhododendron ponticum has several other attributes that allows the species to successfully invade areas. Invasion may take place both vegetatively and by seed dispersal. Established plants are able to spread by horizontal branch growth. Individual plants may cover many square metres with thickly interlaced branches. Horizontal branches may also take root. Over a period of time, a single plant may cover an area of 100 square metres and reach a height  >10 metres.
The species is able to dominate wetlands with their canopies. The roots survive on suitable dry land whilst the lateral stems extend into wet areas. They are capable of completely overgrowing watercourses.
Rhododendron ponticum seeds are small and dispersed by wind. Each flower head may produce thousands of seeds. A large plant is capable of producing several million seeds in a season. Not all seeds germinate and are happier germinating on disturbed ground rather that where there is continuous ground cover.
Rhododendron ponticum leaves are toxic and unpalatable to most herbivores. Young leaf buds exude a sticky substance that traps leaf-eating insects. To add to the list of Rhododendron ponticum woes is the toxicity of honey produced from the flowers. Poisoning from this honey is known as “Mad Honey Disease” or “Honey Intoxication”. The condition is rarely fatal.
There is some evidence that exudate from the species inhibits seed germination of other plants. This theory has yet to be confirmed.
Few native plants survive in Rhododendron ponticum dominated areas. Only the trees above the Rhododendron canopy survive and as they die there are no new recruits because seeds will not germinate under the lightless canopy.
Restoration is both costly and time consuming. Herbicides need to be applied over several years. They do not translocate efficiently throughout the plant. After Rhododendron removal there will be a toxic layer of humus that will prevent native plant regeneration. Removal of this layer is time consuming and re-infestation may occur from the huge seed bank in the soil.
The dramatic effects this single species is having on the British countryside make the influence of our noxious weeds pale into insignificance.
Much of this information was gleaned from the Offwell Woodland and Wildlife Trust web site.